1. Field of the Invention
The present invention relates to a process for producing an aromatic polyamide film excellent in mechanical properties, dimensional stability, and electrical properties. The present invention further relates to an m-phenyleneisophthalamide polymer film having a highly improved dimensional stability under moist conditions.
2. Description of the Prior Art
Aromatic polyamides have excellent thermal properties and mechanical properties. Especially, aromatic polyamides such as p-phenyleneterephthalamide polymers, halogen-substituted p-phenyleneterephthalamide polymers, p-phenyleneisophthalamide polymers, and m-phenyleneisophthalamide polymers are broadly utilized as a material for producing fiber, paper, and the like. However, in spite of the excellent heat resistance and mechanical properties of the aromatic polyamides, aromatic polyamide articles such as films or sheets are inferior to polyimide articles in hygroscopicity and dimensional stability, particularly moisture absorption dimensional stability. Particularly, conventional aromatic polyamide films do not always have satisfactorily high performance, and thus, it has been strongly desired to provide an aromatic polyamide film having improved properties.
For the purpose of attaining the above-mentioned desire, there have been proposed methods in which an m-phenyleneisophthalamide polymer film is treated with a long chain fatty acid, fluorinated or chlorinated aliphatic carboxylic acid, or methyl radical-containing aromatic hydrocarbon; a halogenated aromatic hydrocarbon is incorporated into an m-phenyleneisophthalamide polymer film; or a silicone polymer or fluoropolymer is coated onto the surface of an m-phenyleneisophthalamide polymer film, in Japanese Unexamined Patent Publication (Kokai) Nos. 51-122160, 51-122173, 51-122174, 52-41899, and 52-41900. Also, Japanese Unexamined Patent Publication (Kokai) Nos. 52-56169 and 52-152973 disclose a method for improving the electrical properties of an m-phenyleneisophthalamide polymer film by drawing it biaxially. Further, Japanese Examined Patent Publication (Kokoku) No. 53-45238 discloses a method in which a halogen-substituted p-phenyleneterephthalamide polymer film is heat treated under tension to reduce the hygroscopicity of the film and to improve the electrical properties of the film.
Each of these methods is fairly effective for improving the properties of the aromatic polyamide film. However, these methods can not attain the drastic improvement in the hygroscopicity of an aromatic polyamide owing to the polar amide groups thereof, and thus, the resultant films do not have satisfactorily high performace, having unsatisfactorily improvement in dimensional stability, particularly moisture absorption dimensional stability, and in electrical properties.